IPv6 fc00::/7 — Unique local addresses

Fri Oct 22 17:18:34 UTC 2010

On Fri, 22 Oct 2010 01:10:08 -0700
Owen DeLong <owen at delong.com> wrote:

> 
> On Oct 22, 2010, at 12:55 AM, Mark Smith wrote:
> 
> > On Fri, 22 Oct 2010 15:52:08 +1100
> > Karl Auer <kauer at biplane.com.au> wrote:
> > 
> >> On Thu, 2010-10-21 at 21:05 -0500, Jack Bates wrote:
> >>> On 10/21/2010 8:39 PM, Ray Soucy wrote:
> >>>> 
> >>>> How so? We still have RA (with a high priority) that's the only way
> >>>> DHCPv6 works.  I guess there is a lot of misunderstanding about how
> >>>> DHCPv6 works, even among the experts...
> >>> 
> >>> Actually, the last I checked, there are implementation of DHCPv6 without RA.
> >> 
> >> I'll go out on a limb here and say that RA is not needed for DHCPv6.
> >> 
> > 
> > RAs are still needed to convey the M/O bit values, so that end-nodes
> > know they need to use DHCPv6 if necessary. As there are two address
> > configuration methods, there is always going to be a need to express a
> > policy to end-nodes as to which one they need to use.
> > 
> You can actually force a client to assume the M bit if you cause it to launch
> a DHCPv6 client through other means. You don't have to have RA for that.
> Policy can be expressed by RA, or, it can be expressed by other means.
> 

We used to hand wind cars to start them. We don't have to anymore.

An RA is single, periodic, in the order of 100s of seconds, multicast
packet. If you're arguing against the cost of that, then I think you're
being a bit too precious with your packets.

Any argument for manual configuration is an argument for increasing
complexity and opportunity for error.

> >> A DHCPv6 client multicasts all its messages to the well-known
> >> all-relays-and-servers address. A client needs only its link-local
> >> address to do this. The relay (or server if it happens to be on the same
> >> link) can thus talk to the client in the complete absence of RA.
> >> 
> > 
> > There isn't a method to specify a default gateway in DHCPv6. Some
> > people want it, however it seems a bit pointless to me if you're going
> > to have RAs announcing M/O bits anyway - you may as well use those RAs
> > to announce a default router too.
> > 
> Actually, it's not pointless at all. The RA system assumes that all routers
> capable of announcing RAs are default routers and that virtually all routers
> are created equal (yes, you have high/medium/low, but, really, since you
> have to use high for everything in any reasonable deployment...)
> 

No it doesn't. You can set the router lifetime to zero, which indicates
to the end-node that the RA isn't announcing a default router. In this
case, it may be announcing M/O bit, prefix or other parameters.

> There are real environments where it's desirable to have a way to tell
> different clients on a network to use different default gateways or
> default gateway sets.
> 
> Owen
> 